1. Field of the Invention
This invention relates to implantable cardiac pacemakers and, more particularly, an AAD or VVD mode pacemaker adapted to inhibit or trigger depending upon the timing of each sensed signal.
2. Description of the Prior Art
Implantable pacemakers have developed to the point where a wide variety of different pacemaker modes and functions are available. Relatively early in the development of implantable cardiac pacemakers, it was recognized that it was desirable to permit an underlying natural cardiac beat to occur without pacemaker interference. Thus, pacemakers were developed to function in the inhibit, or demand mode, wherein if a natural heartbeat is detected within the pacemaker escape interval, the pacemaker timing is reset and no pace pulse is delivered. The inhibited mode thus saves battery energy and prevents competition of a delivered pace pulse with a natural cardiac rhythm. However, it was recognized that the demand pacemaker could be vulnerable to false sensing, i.e., it could inhibit upon the sensing of noise or other artifacts instead of a true natural beat. In such an event, the pacemaker would fail to deliver a timely stimulus, with detrimental consequences. An answer to this possibility is to operate in the triggered mode, whereby a sensed event triggers the pacemaker to deliver a pace pulse immediately following the sensed event. In this way, the pacemaker ensures that it is not inhibited due to a false sensed event, and does not compete with the underlying heartbeat. Of course, the trigger mode carries the liability of expending extra battery energy due to delivering heartbeats when there is no need to do so. A ventricular pacemaker which operates in the inhibit mode is classified as VVI; a ventricular pacemaker which operates in the trigger mode classified as VVT; and a ventricular pacemaker which can operate in either mode is classified as VVD. Similarly, an atrial pacemaker may be either AAI, AAT, or AAD.
Most present day pacemakers operate in the inhibit mode. Improvements in amplifier filtering and techniques for recognizing the natural heartbeat waveform to the exclusion of noise and extraneous signals normally permit a high degree of reliability in sensing only the true heartbeat signals. However, present day circuitry still does not provide 100% protection against low rate noise, such as emanates from the patient's muscle. Normally, everything below about 13 Hz is sensed in pacemakers, such that there remains a need to deal with the possibility of low frequency noise sensing. For this reason, there exists a need for an improved pacemaker design which can provide an optimum VVD or AAD mode of operation.